This application claims the priority of German Application No. 102 03 310.2 filed Jan. 29, 2002, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a system for measuring gaseous constituents of a flowing gas mixture. The gas mixture moves in a gas flow control device and comes in contact with a sensor inserted there, which sensor detects individual gas constituents.
From German Patent Document DE 297 06 003 U1, an arrangement is known for measuring a flowing gas. This document describes a xenon gas sensor for monitoring and controlling the xenon concentration in flowing media. This xenon gas sensor has a thermal conductivity sensor and a hat-shaped covering of the sensor resistor. The oncoming flow should not take place in the direction of the opening of the hat-shaped covering of the thermal conductivity sensor.
It is an object of the invention to construct and arrange a system for the determination of the concentration of individual constituents of a flowing gas mixture such that the actual momentary concentration of the constituents to be measured is detected within the gas mixture.
According to the invention, the object is achieved in that at least one mixing device is inserted in the flow of the gas mixture, which mixing device homogenizes the gas mixture to mixed gas before it is detected by the sensor. As a result, it is achieved that filaments of flow existing in the gas mixture are swirled and mixed. In filaments of flow of turbulent or laminar flows, local absolute concentration fluctuations occur with respect to the actual momentary average value, which concentration fluctuations may well be in the one-digit percentage range (10−2) or above. These local below-average or above-average momentary concentrations of different constituents of the gas mixture are compensated by the mixing device and homogenized mixed gas is fed to the sensor and faulty measurements are thereby avoided.
In exhaust gases of internal-combustion engines, such undesirable concentration fluctuations caused by filaments of flow occur more or less in the entire exhaust gas flow control device. Depending on the point at which, for example, a so-called lambda probe is situated as the sensor in the exhaust gas flow control device, filaments of flow are swirled by the system according to the invention. The momentary concentration of a constituent determined when the mixed gas is flowing therefore corresponds with a higher probability than without the mixing device to the absolute momentary concentration. The mixing device according to the invention should basically be applied at an arbitrary point in the exhaust gas flow control device, preferably in the front pipe, behind the high-performance header, in an exhaust muffler or in a section of the exhaust pipe. The lambda probe may be inserted particularly in front of or behind a catalyst or between two catalysts in the exhaust gas flow control device and the mixing device may be provided there.
For this purpose, it is advantageous that the mixing device homogenizes the entire portion or an approximately representative portion of the gas mixture flowing in the exhaust gas flow control device. In the most favorable case, a homogenizing of only 1% of the gas mixture may be sufficient; however, preferably approximately 20% are homogenized. For this purpose, the concentration of the individual constituents may be assumed, for example, to be approximately symmetrical along the cross-section of the gas flow control device. As a result, it may be sufficient to mix only a portion of the gas mixture which is to be considered as representative of the entire cross-section. After the mixing of a portion or of the entire gas mixture, the mixed gas will be accessible to the sensor. If only a portion of the gas mixture is mixed, the mixing device feeds the mixed gas to the sensor. Even when only a portion of the gas mixture is mixed, it may be advantageous to again feed only a portion or the entire mixed gas to the sensor. The quantity of the mixed gas fed to the sensor is, for example, also a function of the frequency at which the sensor measures and is preferably in the range of between 10% and 90%. If the entire gas mixture is detected by the mixing device, there is the possibility to mount the sensor at least behind the mixing device. In this case, it does not have to be provided directly behind the mixing device.
According to a further development of certain preferred embodiments of the invention, one possibility for mixing a portion of the gas mixture flow consists of providing the mixing device only in the center area in the direction of the diameter of the gas flow control device. Here, the mixing device projects from the pipe wall of the gas flow control device into the area of the center axis of the gas flow control device. In this case, the mixing device may have a distance from the opposite pipe wall or extend over the entire diameter. Another possibility consists of provided the mixing device along the entire inner circumference of the gas flow control device. Independently of the positioning and design of the mixing device, the latter forms a type of throttle in the gas flow control device. In this case, part of the pressure loss is utilized for the feeding of the mixed gas to the sensor.
Furthermore, it is advantageous according to certain preferred embodiments of the invention, that the cross-sectional surface of the mixing device varies perpendicularly to the flow direction. As a result, the range of the gas mixture removed for the measurement can be varied. The cross-sectional surface amounts to between 5% and 90%, preferably 15% of the flow cross-section of the gas flow control device. The cross-sectional surface is constructed to be either symmetrical or asymmetrical. Independently of its cross-sectional surface, the mixing device can be inserted directly or in an approximately radial direction through the housing wall of the gas flow control device. During the direct insertion, the mixing device is inserted with or without the sensor in the axial direction into the gas flow control device or is connected by means of an adapter piece into the flow.
It is also advantageous in this regard that the mixing device has at least one inlet opening and at least one outlet opening. The gas mixture entered through the inlet opening is deflected by means of a guiding device in its flow direction and is guided to the sensor. The outlet opening is provided directly at the sensor or in the proximity of the sensor. The gas mixture homogenized by the deflection is for the most part guided past the sensor and is analyzed or detected. The deflection of the flow direction causes the mixing of the gas mixture.
According to a preferred embodiment of the solution according to the invention, it is finally provided that the cross-sections of the inlet openings and outlet openings vary in their size and shape. It can be determined by a series of tests which opening cross-sections are advantageous for which flow parameters, such as the speed, the mass flow rate, the temperature and the composition.
The dimensioning of the mixing device, its shape and its arrangement also have an influence on the shape and position of the inlet and outlet openings. Not only the cross-sectional surface of the mixing device transversely to the flow direction, but also the cross-sections of the mixing device parallel to the flow direction may vary in their size and shape.
The mutual spacing of the inlet openings called the inlet spacing also has an influence on the finally determined measuring result. By varying the inlet spacing, the reliability of the sensor measurement can be improved, for example, when different temperature zones exist within the gas mixture.
It is particularly important according to certain preferred embodiments of the present invention that the size, the shape and the inlet spacing of the inlet and outlet openings with respect to one another varies as a function of the respective distance from the sensor. Preferably, the inlet and outlet openings become smaller with a decreasing distance from the sensor.
As an alternative to the forming of large inlet or outlet openings, these are formed by pores of the mixing device. For this purpose, sintered metals can be used which consist of metal or of a combination of ceramics and metal. Mixing devices are also provided which are formed exclusively of ceramics.
In connection with the construction and arrangement according to certain preferred embodiments of the invention, it is advantageous that the mixing device receives the sensor and is inserted into the gas flow control device together with the sensor. In this case, the sensor can be exchanged alone or together with the mixing device. The exchange takes place by way of an opening in the pipe wall of the gas flow control device or, depending on the insertion, by means of an intermediate piece of a pipe or similar device which is inserted as an adapter into the flow system.
It is also advantageous according to certain preferred embodiments of the invention that the mixing device, in addition to the guiding device, has at least one baffle plate situated on the outside. The baffle plate enlarges the detected flow cross-section and guides more gas mixture to the inlet openings. When used in an exhaust gas flow, the baffle plate heats up significantly faster than the mixing device and thus avoids the forming of condensate in the mixing device. Furthermore, as a result, the mixing device itself is heated more rapidly.
In this context, it is also advantageous according to certain preferred embodiments of the invention that the mixing device can additionally be heated by a heating device. The heating is independent of the use of a baffle plate. For the purpose of the heating, the mixing device accommodates an electrical heating device. In this case, the sensor is positioned such that it is not influenced by possible condensate and by the heating. By means of the heating device, the condensate occurring in or on the mixing device and on the sensor heats up and evaporates. The heating device can advantageously be started up already with the engine start or at least before the dew point of the exhaust gas flow has been reached. The formation of condensate is therefore prevented from the time of the start of the operation. According to certain preferred embodiments of the invention, the heating device is constructed as a grid-type heater, a wall heater or a rod-type heater inside the mixing device.
Additional advantages and details of the invention are explained in the claims and in the specification and are illustrated in the figures.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.